Rear electrode structure for use in photovoltaic device such as CIGS/CIS photovoltaic device and method of making same

ABSTRACT

A photovoltaic device including a rear electrode which may also function as a rear reflector. In certain example embodiments of this invention, the rear electrode includes a metallic based reflective film that is oxidation graded, so as to be more oxided closer to a rear substrate (e.g., glass substrate) supporting the electrode than at a location further from the rear substrate. In other words, the rear electrode is oxidation graded so as to be less oxided closer to a semiconductor absorber of the photovoltaic device than at a location further from the semiconductor absorber in certain example embodiments. In certain example embodiments, the interior surface of the rear substrate may optionally be textured so that the rear electrode deposited thereon is also textured so as to provide desirable electrical and reflective characteristics. In certain example embodiments, the rear electrode may be of or include Mo and/or MoO x , and may be sputter-deposited using a combination of MoO x  and Mo sputtering targets.

This is a continuation-in-part (CIP) of U.S. patent application Ser.Nos. 11/892,161, filed Aug. 20, 2007, and 11/808,764, filed Jun. 12,2007, the entire disclosures of which are hereby incorporated herein byreference.

This invention relates to a rear electrode for use in a photovoltaicdevice or the like, and methods of making the same. The rear (or back)electrode may also function as a rear (or back) reflector in certainexample instances. In certain example embodiments of this invention, therear electrode comprises a metallic based reflective film that isoxidation graded, so as to be more oxided closer to a rear substrate(e.g., glass substrate) supporting the electrode than at a locationfurther from the rear substrate. In other words, the rear electrode isoxidation graded so as to be less oxided closer to a semiconductorabsorber of the photovoltaic device than at a location further from thesemiconductor absorber. In certain example embodiments, there isprovided a method of making the rear electrode for CIS and/or CIGSphotovoltaic (e.g., solar cell) devices using magnetronsputter-deposition of molybdenum (Mo) in a multi-layer configuration. Incertain example embodiments, nitrogen and/or hydrogen gases are used asadditives to the sputtering gas (e.g., argon) to reduce stress of thecoating, enhance its resistance to the selenization during thedownstream device processing, and promote beneficial sodium migrationfrom the soda-lime rear glass substrate to the semiconductor film of thedevice.

BACKGROUND AND SUMMARY OF EXAMPLE EMBODIMENTS OF INVENTION

Photovoltaic devices are known in the art (e.g., see U.S. PatentDocument Nos. 2004/0261841, 2006/0180200, 6,784,361, 6,288,325,6,613,603, and 6,123,824, the disclosures of which are herebyincorporated herein by reference). Examples of photovoltaic devicesinclude CIGS (Cu(In, Ga)(Se, S)₂), CIS (CuInSe₂), and a-Si (amorphoussilicon) solar cells. The CIGS and CIS films are conductivesemiconductor compounds, and these stoichiometries are approximations.

Generally speaking, CIGS and CIS type photovoltaic devices include, fromthe front or light incident side moving rearwardly, a front substrate ofa material such as glass, a front electrode comprising a transparentconductive layer such as a TCO (transparent conductive oxide), a lightabsorption semiconductor film (e.g., CIGS and/or CIS film), a rearelectrode, and a rear substrate of a material such as glass. Sometimesan adhesive is provided between the front substrate and the frontelectrode, and it is also possible for window layer(s) (e.g., of orincluding CdS, ZnO, or the like) to be provided. Photovoltaic power isgenerated when light incident from the front side of the device passesthrough the front electrode and is absorbed by the light absorptionsemiconductor film as is known in the art.

A metal such as Mo (molybdenum) may be used as the rear electrode(bottom contact) of a photovoltaic device, such as a CIS solar cell. Incertain instances, the Mo may be sputter-deposited onto a soda orsoda-lime-silica rear glass substrate of the photovoltaic device.However, Mo rear electrodes suffer from the problem of delamination fromthe rear substrate.

Rear electrodes (e.g., Mo rear electrodes) preferably have low stress,high conductivity, and good adhesion to the rear substrate (e.g., glasssubstrate). In order to provide this combination of features, oxygen isintroduced into the Mo rear electrode at the initial stage of depositionof the rear electrode on the substrate or otherwise in certain exampleembodiments of this invention. The application of the oxygen to the Morear electrode reduces the overall stress of the rear electrode and atthe same time promotes adhesion of the rear electrode to the glass sodaor soda lime silica glass substrate.

In certain example embodiments of this invention, there is provided amethod of making the rear electrode for CIS and/or CIGS photovoltaic(e.g., solar cell) devices using magnetron sputter-deposition ofmolybdenum (Mo) in a multi-layer configuration. In certain exampleembodiments, nitrogen and/or hydrogen (e.g., H₂) gas(es) are used asadditives to the sputtering gas (e.g., argon) to reduce stress of thecoating, enhance its resistance to the selenization during thedownstream device processing, and promote beneficial sodium migrationfrom the soda-lime rear glass substrate to the semiconductor film of thedevice.

In certain example embodiments, there is provided a method of making arear electrode structure for a photovoltaic device, the methodcomprising: providing a glass substrate; depositing a conductiveelectrode comprising Mo (molybdenum) on the glass substrate; and whereinsaid depositing the conductive electrode comprising Mo (molybdenum)comprises sputtering at least one target comprising Mo (metallic Mo or aMoOx ceramic in example embodiments) in an atmosphere including (i) aninert gas such as argon or the like, and (ii) from about 0.1 to 10%nitrogen and/or hydrogen gas.

In certain example embodiments of this invention, there is provided amethod of making a rear electrode structure for a photovoltaic device,the method comprising: providing a glass substrate; depositing aconductive electrode comprising Mo (molybdenum) on the glass substrate;and wherein said depositing the conductive electrode comprising Mo(molybdenum) comprises sputtering at least one ceramic target comprisingMoO_(x) and at least one metallic target comprising Mo in depositing theconductive electrode.

In certain example embodiments of this invention, there is provided amethod of making a rear electrode structure for a photovoltaic device,the method comprising: providing a substrate (glass or any othersuitable material); depositing a conductive electrode comprising a metal(M) on the substrate; and wherein said depositing the conductiveelectrode comprises sputtering at least one ceramic target comprisingMO_(x) and at least one metallic target comprising M in depositing theconductive electrode. The metal M may be Mo or any other suitable metal.

In other example embodiments of this invention, there is provided amethod of making a rear electrode structure for a photovoltaic device,the method comprising: providing a glass substrate; depositing aconductive electrode comprising Mo (molybdenum) on the glass substrate;and wherein said depositing the conductive electrode comprising Mo(molybdenum) comprises sputtering at least one ceramic target comprisingMoO_(x), where x is less than or equal to 0.1.

In other example embodiments of this invention, there is provided aphotovoltaic device comprising: a front substrate; a front substantiallytransparent conductive electrode; an absorber semiconductor film; aconductive and reflective rear electrode; a rear glass substrate thatsupports at least the rear electrode; and wherein the rear electrodecomprises a first layer or layer portion comprising an oxide of Mo and asecond conductive layer or layer portion comprising substantiallymetallic Mo provided on the rear glass substrate over at least the firstlayer, so that the first layer or layer portion comprising the oxide ofMo is located between the rear glass substrate and the second layer orlayer portion comprising substantially metallic Mo.

In certain example embodiments, the rear electrode is formed in a mannerso that its major surface to be closest to the light absorptionsemiconductor film of the photovoltaic device is textured (roughened) ina substantially controlled and desired manner. In certain exampleembodiments, the interior surface of the rear glass substrate istextured (roughened) via acid etching, ion beam treatment, or the like.Then, the Mo inclusive rear electrode is formed on the textured surfaceof the rear glass substrate in a manner so that the major surface of therear electrode to be closest to the light absorption semiconductor filmis also textured. In certain example embodiments, the textured majorsurface of the rear electrode to be closest to the light absorptionsemiconductor film may be substantially conformal to the texturedsurface of the rear glass substrate.

The embodiments where the rear substrate is textured may or may not beused in combination with the embodiments where the rear electrode isoxidation graded, in different example embodiments of this invention.

The textured interior surface of the rear electrode is advantageous inseveral example respects. The textured surface of the rear electrodeimproves adhesion between the rear electrode and the semiconductor film.Moreover, the textured surface of the rear electrode allows the rearelectrode to act as a scattering back electrode thereby permitting it toreflect incident light more effectively and efficiently into the lightabsorption semiconductor film. This can allow one of both of: improvedefficiency of the photovoltaic device, and/or reduced thickness of thelight absorption semiconductor film without sacrificing solarefficiency. In certain example embodiments, after the rear electrode hasbeen formed on the rear glass substrate, the major surface of the rearelectrode to be closest to the light absorption semiconductor film maybe textured via one or more of ion beam treatment, plasma exposure,and/or a wet chemical etch such as HCl, nitric acid, acetic acid or acombination thereof. This post-deposition texturing (roughening) of therear electrode surface may, or may not, be used in combination withembodiments where the rear glass substrate is textured. Thus, when therear electrode is textured (roughened) after the deposition thereof, therear glass substrate may or may not be textured. The textured rear glasssubstrate and/or textured rear electrode (which also functions as areflector) may be used in a photovoltaic device (e.g., CIS or CIGS solarcell) where an active semiconductor film is provided between the rearelectrode/reflector and a front electrode(s).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a) and 1(b) are cross sectionals view of example photovoltaicdevices according to example embodiments of this invention.

FIG. 2 is a cross-sectional view of an example rear electrode and rearglass substrate that may be used in the photovoltaic device of FIG. 1(b) in an example embodiment of this invention.

FIG. 3( a) is a cross-sectional view of another example rear electrodeand rear glass substrate that may be used in the photovoltaic device ofFIG. 1( b) in an example embodiment of this invention.

FIGS. 3( b) and 3(c) are cross-sectional views of other example rearelectrodes and corresponding rear glass substrates that may be used inthe photovoltaic device of FIG. 1( a) in example embodiments of thisinvention.

FIG. 4 is a cross-sectional view of another example rear electrode andrear glass substrate that may be used in the photovoltaic device of FIG.1( a) or 1(b) in example embodiments of this invention.

FIG. 5 is a cross-sectional view of another example rear electrode andrear glass substrate that may be used in the photovoltaic device of FIG.1( a) or 1(b) in example embodiments of this invention.

FIG. 6 is a flowchart illustrating how a rear electrode structure and/orphotovoltaic device may be made according to an example embodiment ofthis invention (e.g., with respect to FIGS. 1( b), 2, 3(a), and 4-5).

FIG. 7 is a flowchart illustrating how a rear electrode structure and/orphotovoltaic device may be made according to another example embodimentof this invention (e.g., with respect to FIGS. 1( b), 2, 3(a), and 4-5).

FIG. 8 is a schematic diagram illustrating an example of how anoxidation graded Mo inclusive rear electrode may be made for use in anyof the FIG. 1-7 embodiments according to example embodiments of thisinvention.

FIG. 9 is a schematic diagram illustrating another example of how anoxidation graded Mo inclusive rear electrode may be made for use in anyof the FIG. 1-7 embodiments according to example embodiments of thisinvention; different types of sputtering targets are used in FIG. 9compared to FIG. 8.

FIG. 10 is a cross-sectional view of another example rear electrode andrear glass substrate that may be used in a photovoltaic device of anyembodiment herein, according to examples of this invention.

FIG. 11 is a percent nitrogen gas flow vs. resistivity graphillustrating how the introduction of nitrogen gas into the sputteringprocess for the rear electrode in certain example embodiments of thisinvention affects conductivity (and thus resistivity) of the rearelectrode.

FIG. 12 is an energy diagram of an example CIS photovoltaic device.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

Referring now more particularly to the figures in which like referencenumerals refer to like parts/layers in the several views.

Photovoltaic devices such as solar cells convert solar radiation intousable electrical energy. The energy conversion occurs typically as theresult of the photovoltaic effect. Solar radiation (e.g., sunlight)impinging on a photovoltaic device and absorbed by an active region ofsemiconductor material (e.g., a semiconductor film including one or moresemiconductor layers such as a-Si, CIS, CIGS or the like, thesemiconductor sometimes being called an absorbing layer or film)generates electron-hole pairs in the active region. The electrons andholes may be separated by an electric field of a junction in thephotovoltaic device. The separation of the electrons and holes by thejunction results in the generation of an electric current and voltage.In certain example embodiments, the electrons flow toward the region ofthe semiconductor material having n-type conductivity, and holes flowtoward the region of the semiconductor having p-type conductivity.Current can flow through an external circuit connecting the n-typeregion to the p-type region (or vice versa) as light continues togenerate electron-hole pairs in the photovoltaic device.

In certain example embodiments, single junction amorphous silicon (a-Si)photovoltaic devices include three semiconductor layers. In particular,the semiconductor film includes a p-layer, an n-layer and an i-layerwhich is intrinsic. The amorphous silicon film (which may include one ormore layers such as p, n and i type layers) may be of hydrogenatedamorphous silicon in certain instances, but may also be of or includehydrogenated amorphous silicon carbon or hydrogenated amorphous silicongermanium, or the like, in certain example embodiments of thisinvention. For example and without limitation, when a photon of light isabsorbed in the i-layer it gives rise to a unit of electrical current(an electron-hole pair). The p and n-layers, which contain chargeddopant ions, set up an electric field across the i-layer which draws theelectric charge out of the i-layer and sends it to an optional externalcircuit where it can provide power for electrical components. In certainother example embodiments of this invention, the absorptionsemiconductor film may be of or include CIGS (approximately Cu(In,Ga)(Se, S)₂) and/or CIS (approximately CuInSe₂). However, it is notedthat while certain example embodiments of this invention are directedtoward CIGS, CIS and/or amorphous-silicon based photovoltaic devices,this invention is not so limited and may be used in conjunction withother types of photovoltaic devices in certain instances including butnot limited to devices including other types of semiconductor material,single or tandem thin-film solar cells, CdS and/or CdTe photovoltaicdevices, polysilicon and/or microcrystalline Si photovoltaic devices,and the like.

In certain example embodiments of this invention, there is provided amethod of making the rear opaque electrode for CIS and/or CIGSphotovoltaic (e.g., solar cell) devices using magnetronsputter-deposition of molybdenum (Mo) in a multi-layer configuration. Incertain example embodiments, nitrogen and/or hydrogen gases are used asadditives to the sputtering gas (e.g., argon) to reduce stress of thecoating, enhance its resistance to the selenization during thedownstream device processing, and promote beneficial sodium migrationfrom the soda-lime rear glass substrate to the semiconductor film of thedevice. Moreover, oxygen may be introduced at least in an area close tothe rear glass substrate for improvement of durability and the like.

FIGS. 1( a) and 1(b) are cross sectional views of photovoltaic devicesaccording to example embodiments of this invention, whereas FIGS. 2-5are cross-sectional views of example rear electrodes and correspondingrear glass substrates that may be used in the photovoltaic device(s) ofFIG. 1( a) and/or 1(b) in example embodiments of this invention.

Referring to FIGS. 1( a) and 1(b), the photovoltaic devices includetransparent front glass substrate 1, optional adhesive film 2, singlelayer or multilayer front conductive electrode 3, active semiconductorfilm 5 of or including one or more semiconductor layers (such as CIGS,CIS, a-Si, or the like), electrically conductive backelectrode/reflector 10, and rear glass substrate 11. Rear electrode 10is preferably continuous or substantially continuous across all or asubstantial portion of glass substrate 11, although it may be patternedinto a desired design (e.g., stripes) in certain instances. The optionaladhesive 2 may be of or include an electrically insulating polymer basedand/or polymer inclusive encapsulant or adhesive of a material such asethyl vinyl acetate (EVA), polyvinyl butyral (PVB), or the like. Incertain example embodiments, polymer based adhesive layer 2 has arefractive index (n) of from about 1.8 to 2.2, more preferably fromabout 1.9 to 2.1, with an example being about 2.0, for purposes ofenhancing internal reflection if textural back glass is used. Of course,other layer(s) which are not shown may also be provided in the device.For instance, buffer and/or window layer(s) may also optionally beprovided.

Front glass substrate 1 and/or rear glass substrate 11 may be made ofsoda-lime-silica based glass in certain example embodiments of thisinvention; and front glass substrate 1 may have low iron content (e.g.,less than about 0.10% total iron, more preferably less than about 0.08,0.07 or 0.06% total iron) and/or an antireflection coating (not shown)thereon to optimize transmission in certain example instances. Whilesubstrates 1, 11 may be of glass in certain example embodiments of thisinvention, other materials such as quartz, plastic or the like mayinstead be used for substrate(s) 1 and/or 11. Glass substrate(s) 1and/or 11 may or may not be thermally tempered in certain exampleembodiments of this invention. Additionally, it will be appreciated thatthe word “on” as used herein covers both a layer being directly on andindirectly on something, with other layers possibly being locatedtherebetween.

Referring to FIGS. 1-5, a metal such as Mo (molybdenum) may be used asthe rear electrode (bottom contact) 10 of a photovoltaic device, such asa CIS solar cell. In certain instances, the Mo may be sputter-depositedonto a soda or soda-lime-silica rear glass substrate 11 of thephotovoltaic device. However, pure Mo rear electrodes suffer from theproblem of delamination from the rear substrate 11.

Rear electrodes (e.g., Mo rear electrodes) 10 preferably have lowstress, high conductivity, and good adhesion to the rear substrate(e.g., glass substrate) 11. In order to provide this combination offeatures, oxygen is introduced into the Mo based rear electrode 10 atthe initial stage of deposition of the rear electrode on the substrate11 or otherwise in certain example embodiments of this invention. Theapplication of the oxygen to the Mo-based rear electrode 10 reduces theoverall stress of the rear electrode and at the same time promotesadhesion of the rear electrode 10 to the glass soda or soda lime silicaglass substrate 11. However, in certain large sputter coaters designedfor large substrate widths such as greater than one meter, it issometimes difficult to control the uniformity of oxygen in the finalrear electrode film due to the different pumping speeds between reactivegas (e.g., oxygen) and sputtering gas (e.g., Ar).

FIGS. 8-9 are schematic diagrams illustrating example techniques ormethods for sputter-depositing rear electrodes 10 on substrates 11(directly or indirectly) in certain example embodiments of thisinvention. As shown in FIGS. 8-9, in certain example embodiments of thisinvention, instead of using only metallic Mo targets 60 tosputter-deposit the Mo-based rear electrode 10 on the rear substrate 11,ceramic MoO_(x) (where x is from 0.001 to 0.25, more preferably fromabout 0.01 to 0.2, even more preferably from about 0.01 to 0.15, andmost preferably from about 0.03 to 0.10) target(s) 70 are also orinstead used. The ceramic MoO_(x) sputtering target(s) 70 may or may notbe used in combination with metallic Mo sputtering target(s) 60 incertain example embodiments of this invention. By using ceramic MoO_(x)sputtering target(s) 70 at least during the initial deposition phase ofthe rear electrode 10, little or no oxygen is introduced into theatmosphere surrounding the target during sputtering and thus the contentand/or stoichiometry of the resulting thin film rear electrodecomposition is easier to control. In certain example embodiments only Argas (or other inert gas) is introduced proximate the Mo inclusivetarget(s) 60 and/or 70 during sputter-deposition of the rear electrode10. In other words, in certain example embodiments little or no oxygengas is introduced proximate the Mo inclusive targets 60 and 70 duringsputtering thereof in forming the rear electrode. In certain exampleembodiments, H₂ gas or any other suitable gas may also be introducedalong with the Ar gas in order to further reduce oxygen concentration inthe rear electrode. In other words, the final film composition ofelectrode 10 is controlled more or mainly by the target(s) compositionin at least part of the thin film rear electrode 10.

Still referring to FIGS. 8-9, in certain example embodiments, by using aco-sputtering approach utilizing at least adjacent targets, one or moreof metallic Mo 60 and the other or more of ceramic MoO_(x) 70, an oxygengraded composition through at least part of the thickness of theMo-based rear electrode 10 may be provided as shown for example in FIGS.1( a), 3(a), 3(b), 3(c), 4 and 5. This is advantageous in that theoxidation grading has been found to enhance durability of the rearelectrode 10 and thus of the photovoltaic device. As shown in FIGS. 1(a), 3(a), 3(b), 3(c), 4 and 5, due to the arrangement of MoO_(x) and Motargets in FIGS. 8-9, the rear electrode 10 is oxidation graded(continuously or discontinuously) in a manner so that the electrode 10is more oxided at a location therein closer to the rear substrate 11than at a location therein farther from the rear substrate 11.

The MoO_(x) and Mo targets 70 and 60, respectively, used inco-sputtering or co-deposition may be rotating sputtering target(s) (seethe round targets in FIGS. 8-9) and/or stationary planar target(s) (seethe planar shaped targets in FIGS. 8-9). Such target(s) 60 and/or 70 maybe fabricated by casting, isostatic press, extrusion, or spraying indifferent example embodiments of this invention.

While the targets 60 and 70 are Mo-based, and the rear electrode 10 isMo-based in the illustrated embodiments of this invention, thisinvention is not so limited. Other metal(s) (M) may be used instead ofMo in the rear electrode 10 and/or in the target(s) 60, 70.

In the FIG. 1( a) embodiment of this invention, the Mo-based rearelectrode (which may be oxidation graded as discussed herein) issupported by a substantially flat surface of the rear substrate 11.However, in other example embodiments (e.g., see FIGS. 1( b), 2, 3(a), 4and 5), the rear electrode may be formed on a textured surface of therear substrate 11.

With respect to FIGS. 1( b), 2, 3(a), 4 and 5, the interior surface ofthe rear glass substrate 11 is macroscopically textured in order toimprove reflective scattering, and the rear electrode 10 (which may ormay not be oxidation graded as discussed herein) is deposited (e.g., viasputtering or the like) on the textured surface 11 a of the substrate11. Rear electrode 10 is thus able to reflect significant amounts oflight in the 500-800 nm, and/or 600-800 nm wavelength range, therebypermitting such light to be trapped or to go through a long path in thesemiconductor film 5 to enhance the photovoltaic efficiency of thedevice. In certain example embodiments, the macroscopically texturedinterior surface 11 a of glass substrate 11 may have any suitabletextured pattern, such as a pyramid pattern obtained by rolling or thelike, a random pattern achieved by ion beam treatment, rolling, and/oracid etching. This textured pattern may have a periodicity of from about20 to 200 μm (more preferably from about 40 to 100 μm) in certainexample embodiments, depending on the capabilities of the glasspatterning line, ion beam treatment, and/or etching process. Otherpossible patterns for the interior surface 11 a of glass 11 includetriangular or sawtooth trough patterns and, in general, any combinationof slanted patterns which maximizes or substantially maximizes multipleinternal reflections.

In certain example embodiments of this invention, rear electrode 10comprises Mo (molybdenum) and is provided on, directly or indirectly,rear glass substrate 11. The Mo inclusive rear electrode 10 is formed ina manner so that its major surface 10 a to be closest to the lightabsorption semiconductor film 5 is textured (roughened) in asubstantially controlled and desired manner (e.g., see FIGS. 1( b), 2,3(a), 4 and 5). In certain example embodiments, the interior surface 11a of the rear glass substrate 11 is textured (roughened) via acidetching, ion beam treatment, or the like. Then, after the glasssubstrate 11 is textured, the Mo-based rear electrode 10 is formed onthe textured surface 11 a of the rear glass substrate 11 in a manner sothat the major surface 10 a of the rear electrode 10 to be closest tothe light absorption semiconductor film 5 is also textured. In certainexample embodiments, the textured major surface 10 a of the rearelectrode 10 to be closest to the light absorption semiconductor filmmay be substantially conformal to the textured surface 11 a of the rearglass substrate 11. The textured surface(s) of the rear electrode 10permit(s) the rear electrode to provide both desirable electrical andreflective characteristics.

The textured interior surface 10 a of the rear electrode 10 isadvantageous in several example respects. The textured surface 10 a ofthe rear electrode 10 allows the rear electrode to act as a scatteringback electrode thereby permitting it to reflect incident light (lightwhich has come into the device from the sun or the like and passedthrough the front electrode 3 and film 5) more effectively andefficiently back into the light absorption semiconductor film 5. Thiscan allow one of both of: improved efficiency of the photovoltaicdevice, and/or reduced thickness of the light absorption semiconductorfilm 5 without sacrificing solar efficiency.

Still with respect to FIGS. 1( b), 2, 3(a), 4 and 5, in certain exampleembodiments, after the rear electrode 10 has been formed on the rearglass substrate 11, the major surface of the rear electrode 10 to beclosest to the light absorption semiconductor film 5 may be textured viaone or more of ion beam treatment, plasma exposure, and/or a wetchemical etch such as HCl, nitric acid, acetic acid or a combinationthereof, to form textured surface 10 a. This post-deposition texturing(roughening) of the rear electrode surface 10 a may, or may not, be usedin combination with embodiments where the rear glass substrate istextured. Thus, when the surface 10 a of the rear electrode 10 istextured (roughened) after the deposition of the rear electrode on thesubstrate 11, the rear glass substrate 11 may or may not be textured.

Because of this improved back electrode structure (10 and/or 11) whichprovides improved reflection back into the semiconductor film 5, thefront electrode 3 and/or front substrate 1 need not be textured(although it or they may be in certain instances). Moreover, the lightincident surface 5 a of the semiconductor film 5 need not be textured(although it may be in certain instances). Because the front electrode 3and semiconductor film 5 may be smooth or substantially smooth, thereliability and/or manufacturing yield of the device can be improved,and possibly a thinner semiconductor film 5 may be used in certainexample instances. Moreover, the front electrode 3 may be a multi-layercoating including at least one silver layer or the like to be used toform the front electrode 3 in certain example instances; such coatingsfor electrode 3 may have an improved (e.g., lower) sheet resistancewhile at the same time maintaining high transmission in the part of thespectrum in which the photovoltaic device is sensitive (e.g., 350 to750, 350 to 800 nm, or possibly up to about 1100 nm for certain types).Low sheet resistance is advantageous in that it allows for less denselaser scribing and may lead to lower scribe losses. Furthermore, thetotal thickness of such a multilayer front electrode 3 may be less thanthat of a conventional TCO front electrode in certain examplenon-limiting instances, which can reduce the cost of the product andincrease throughput. Example multi-layer coatings for the frontelectrode 3 are described in U.S. Ser. No. 11/724,326, the disclosure ofwhich is hereby incorporated herein by reference.

FIG. 2 is an enlarged cross-sectional view of an example rear electrodestructure for use in the photovoltaic device of FIG. 1( b) in an exampleembodiment of this invention. FIG. 2 illustrates that the rear electrodestructure includes rear glass substrate 11 which may be substantiallytransparent, and rear electrode 10. In the FIG. 2 embodiment the rearelectrode 10 is of or includes Mo, and may be metallic or substantiallymetallic (possibly oxidation graded as discussed herein) so as to bereflective. In the FIG. 2 embodiment, the textured surfaces 10 a and 11a of the rear electrode 10 and rear glass substrate 11, respectively,have peaks 9 a, valleys 9 b between the peaks, and inclined portions 9 cconnecting the peaks and valleys.

Referring to FIGS. 1( b), 2, 3(a), 4 and 5, incident light from the sunmakes its way first through front substrate 1 and front electrode 3, andinto semiconductor film 5. Some of this light proceeds throughsemiconductor film 5 thereby reaching rear electrode 10, and isreflected by the rear electrode 10 which is provided on the interiortextured surface of the rear substrate 11. It has been found thatespecially good reflection occurs in certain example instances whenreflective rear electrode 10 includes inclined portions 9 c which forman angle(s) a with the plane (and/or rear surface) of the rear substrate11, where α is at least about 20 degrees, more preferably from about25-50 degrees, even more preferably from about 25-40 or 25-35 degrees(note: angle α is shown in FIG. 2 with respect to the inclined portions9 c of the rear substrate, but this also applies to the inclinedportions 9 c of the rear electrode 10). While not shown in FIGS. 3-5 forpurposes of simplicity, this concept regarding angle(s) α is alsoapplicable to those figures. Causing this angle α to be within such arange for at least some inclined portions is advantageous in that morelight is kept in the cell (i.e., in the semiconductor 5 for conversionto current) so that the efficiency of the photovoltaic device isimproved.

In certain example embodiments of this invention (e.g., see FIG. 1), asingle layer front electrode 3 may be used in the device. An examplesingle-layer front electrode 3 may be of a TCO such as tin oxide(possibly fluorinated), zinc oxide, ITO, or the like.

In other example embodiments, a multilayer front electrode 3 may be usedin the photovoltaic device. Example multilayer front electrodes 3 aredescribed in U.S. Ser. Nos. 11/724,326 filed Feb. 27, 2007 and11/591,668 filed Nov. 2, 2006 (both hereby incorporated herein byreference in their entireties), for purposes of example. Such an examplemultilayer front electrode 3 includes from the glass substrate 1 movingtoward the semiconductor film 5, a first transparent conductive oxide(TCO) layer, a first conductive substantially metallic IR reflectinglayer (e.g., of Ag or the like), a second TCO layer, second conductivesubstantially metallic IR reflecting layer (e.g., of Ag or the like), athird TCO layer, and optionally a buffer layer. Optionally, the firstTCO layer may be a dielectric layer instead of a TCO in certain exampleinstances and serve as a seed layer for the first conductivesubstantially metallic IR reflecting layer. Of course, it is possiblefor certain layers of this multilayer front electrode to be removed incertain alternative embodiments of this invention, and it is alsopossible for additional layers to be provided in the multilayer frontelectrode. Front electrode 3 may be continuous across all or asubstantial portion of glass substrate 1 and may be flat in certainexample instances (i.e., not textured), or alternatively may bepatterned into a desired design (e.g., stripes), in different exampleembodiments of this invention. Each of layers/films 1-3 is substantiallytransparent in certain example embodiments of this invention.

The active absorption semiconductor region or film 5 may include one ormore layers, and may be of any suitable material. In certain exampleembodiments of this invention, the absorption semiconductor film 5 is ofor includes ABC_(x), where A is a group IB element(s) such as Cu, Agand/or Au, B is a group IIIA element(s) such as In and/or Ga, C is agroup VI element(s) such as Se and/or S, and x is from about 1.5 to 2.5,more preferably from about 1.9 to 2.1, with an example value of x beingabout 2.0. Thus, the semiconductor film 5 may be of or include CIGS(approximately Cu(In or Ga)(Se or S)₂) and/or CIS (approximatelyCuInSe₂) in certain example embodiments. The active semiconductor film 5may be formed by any suitable technique, including but not limited tovacuum evaporation or the like. Alternatively, the semiconductor film 5may be of or include a-Si or other suitable material in certaininstances.

Rear conductive electrode 10 (sometimes referred to as a back contact)may be oxidation graded and may include one or more layers, andcomprises molybdenum (Mo) in certain example embodiments. Part or all ofthe rear electrode 10 may be oxidation graded in different exampleembodiments of this invention. In certain example embodiments, the rearelectrode 10 is in direct contact with the semiconductor film 5. Rearelectrode 10 may be formed via sputtering or any other suitabletechnique in different example embodiments of this invention.

FIG. 2 is a cross-sectional view of an example conductive andsubstantially opaque rear electrode (and reflector) 10 and rear glasssubstrate 11 that may be used in the photovoltaic device of FIG. 1( b)in an example embodiment of this invention. In the FIG. 2 embodiment,the rear electrode 10 is of a single metallic or substantially metalliclayer of Mo which may or may not be oxidation graded. The conductive andreflective Mo electrode 10 may optionally be doped with small amounts ofother elements in certain instances. The thickness of the Mo electrode10 may be varied depending on the desired sheet resistance of the rearelectrode. In certain example instances, the Mo electrode 10 in the FIG.2 embodiment may be from about 1500 to 5000 Å thick, more preferablyfrom about 2500 to 4500 Å thick.

FIG. 3( a) is a cross-sectional view of an example conductive andsubstantially opaque rear electrode (and reflector) 10 and rear glasssubstrate 11 that may be used in the photovoltaic device of FIG. 1( b)in another example embodiment of this invention, whereas FIGS. 3( b) and3(c) are cross-sectional views of example conductive and substantiallyopaque rear electrode 10 and substrates 11 that may be used in thephotovoltaic devices of FIG. 1( a). Referring to FIGS. 3( a)-(c), therear electrode 10 in this embodiment is the same as that discussed abovewith the following exceptions. In the FIG. 3 embodiment, the rearelectrode 10 includes a first layer or layer portion 10 b of orincluding an oxide of Mo (molybdenum) (e.g., MoO_(x), where in certainexample embodiments x may be from about 0.2 to 1.0, more preferably fromabout 0.5 to 1.0) which may have been sputter-deposited using MoO_(x)target(s) 70 in FIGS. 8-9, and a second metallic or substantiallymetallic layer or layer portion 10 c of or based on Mo that may havebeen sputter-deposited on substrate 11 using the metallic Mo target(s)60 in FIGS. 8-9. The thickness of the Mo layer 10 c may be varieddepending on the desired sheet resistance of the rear electrode 10. Incertain example instances, the Mo layer 10 c may be from about 1500 to5000 Å thick, more preferably from about 2500 to 4500 Å thick, and theMoO_(x) layer 10 b may be from about 50 to 1000 Å thick, more preferablyfrom about 100 to 600 Å thick, and most preferably from about 200 to 300Å thick. While the Mo layer 10 c may be deposited by sputtering a Motarget(s) in certain example embodiments in an atmosphere of argon gasor the like, the MoO_(x) layer 10 b may be deposited by sputtering a Moor MoO_(x) target(s) (e.g., in an argon, or an argon/oxygen gasatmosphere). As shown by FIGS. 3( b)-(c), the oxidation graded Mo-basedrear electrode 10 may be considered a single-layer or multiple-layerelectrode 10 in different instances. In certain example embodiments, theMo based layer 10 c may or may not be oxidation graded, continuously ordiscontinuously, so as to be more metallic at a location therein closerto the semiconductor film 5 than at a location therein closer to thesubstrate 11. These layers 10 b and/or 10 c may be sputter-deposited atroom temperature in certain example embodiments, or also may bedeposited in heat vacuum chamber(s). Optionally, the glass 11 may beheated when one or both of these layers is sputter-deposited in certainexample instances. Heating the glass (e.g., using temperatures of fromabout 150-300 degrees C., more preferably from about 200-250 degrees C.)may be advantageous in that the heat causes a more dense Mo-based layerto be formed thereby resulting in lower sheet resistance for the layer.Thus, for a given sheet resistance, a thinner Mo layer 10 c may be usedin certain example instances.

Still referring to at least the FIG. 3 embodiment (and the FIG. 4-5embodiments where Mo oxide may also be used in the rear electrode), ithas been found that sodium (Na) migration from the rear glass substrate11 improves the performance of CIS and/or CIGS photovoltaic devices. Inparticular, it has been found that sodium migration from the rear glasssubstrate 11 to the surface of the Mo based layer 10 closest to thesemiconductor improves the performance of CIS and/or CIGS photovoltaicdevices. From about 3-5% sodium migration to the surface of the Mo basedelectrode has been found to be particularly beneficial in certainexample instances. Accordingly, it has been found that such sodiummigration from the glass 11 is desired to some extent. It hassurprisingly been found that the presence of the MoO_(x) layer or layerportion 10 b helps accelerate such sodium migration from the rear glasssubstrate 11 to the surface of the Mo based electrode 10 during heattreatment at high temperatures used during processing/manufacturing ofthe device. Thus, it will be appreciated that the use of the MoO_(x)layer 10 b in the rear electrode is highly advantageous in certainexample embodiments of this invention due to its ability to increase Nadiffusion from the glass 11 to the surface of the electrode 10 adjacentthe semiconductor 5 and possibly into the semiconductor 5. The MoO_(x)layer or layer portion 10 b may be conductive, semiconductive, ornon-conductive in different embodiments, depending on the amount ofoxygen provided in the layer. Small amounts of other element(s) may alsobe provided in layer(s) 10 b and/or 10 c in certain instances.

FIG. 4 is a cross-sectional view of an example conductive andsubstantially opaque rear electrode (and reflector) 10 and rear glasssubstrate 11 that may be used in the photovoltaic device of FIG. 1( b)(or FIG. 1( a)) in another example embodiment of this invention. Therear electrode 10 in this embodiment is the same as that discussed abovewith respect to FIGS. 1-3 with the following exceptions. In the FIG. 4embodiment, the rear electrode 10 includes a first metallic orsubstantially metallic layer 10 d of or based on Cr, a second layer orlayer portion 10 b of or including an oxide of Mo (molybdenum) (e.g.,MoO_(x), where in certain example embodiments x may be from about 0.2 to1.0, more preferably from about 0.5 to 1.0), and a third metallic orsubstantially metallic layer or layer portion 10 c of or based on Mo. Incertain example embodiments, the Cr based layer 10 d may be from about50 to 1000 Å thick, more preferably from about 100 to 600 Å thick, andmost preferably from about 200 to 300 Å thick. Example thicknesses of,and deposition techniques for, layers 10 b and 10 c are discussed above.It has been found that the Cr layer 10 d is advantageous in that itpermits better adhesion of the layer 10 b and/or 10 c to the glass 11.In alternatives of the FIG. 4 embodiment, the MoO_(x) layer 10 b may beomitted so that the Cr layer 10 d and the Mo layer 10 c directly contacteach other (this may serve to advantageous reduce the sheet resistanceof the rear electrode in certain example instances). The Cr layer 10 dmay include other element(s) such as Ni and/or oxygen in certain exampleinstances.

FIG. 5 is a cross-sectional view of an example conductive andsubstantially opaque rear electrode (and reflector) 10 and rear glasssubstrate 11 that may be used in the photovoltaic devices of FIG. 1 inanother example embodiment of this invention. The rear electrode 10 inthis embodiment is the same as that discussed above with respect toFIGS. 1-3 with the following exceptions. In the FIG. 5 embodiment, therear electrode 10 includes a first metallic or substantially metalliclayer 10 e of or based on Ti, a second layer or layer portion 10 b of orincluding an oxide of Mo (molybdenum) (e.g., MoO_(x), where in certainexample embodiments x may be from about 0.2 to 1.0, more preferably fromabout 0.5 to 1.0), and a third metallic or substantially metallic layeror layer portion 10 c of or based on Mo. In certain example embodiments,the Ti based layer 10 e may be from about 50 to 1000 Å thick, morepreferably from about 100 to 600 Å thick, and most preferably from about200 to 300 Å thick. Example thicknesses of, and deposition techniquesfor, layers 10 b and 10 c are discussed above. It has been found thatthe Ti layer 10 d is advantageous in that it permits better adhesion ofthe layer 10 b and/or 10 c to the glass 11. In alternatives of the FIG.5 embodiment, the MoO_(x) layer 10 b may be omitted so that the Ti layer10 e and the Mo layer 10 c directly contact each other (this may serveto advantageous reduce the sheet resistance of the rear electrode incertain example instances). The Ti layer 10 e may include otherelement(s) such as nitrogen and/or oxygen in certain example instances.

It is of course possible for other layers to be provided, or for certainlayers to be omitted, in other example embodiments of this invention.

FIG. 6 is a flowchart illustrating steps taken in making a photovoltaicdevice according to an example embodiment of this invention relating totextured rear substrates as shown in FIGS. 1( b), 2, 3(a), 4 and 5. Rearglass substrate 11 (e.g., soda-lime-silica based glass) is provided. Thesurface of the glass substrate to be closest to the semiconductor 5 istextured (roughened) as discussed above using any suitable technique,such as via ion beam treatment and/or acid etching (S1). If ion beamtreatment is used, example ion sources and ion beam treating techniquesare described in U.S. Pat. Nos. 7,049,003 and 6,878,403, the disclosuresof which are hereby incorporated herein by reference. Then, the rearelectrode 10 is deposited (e.g., via sputtering or the like) on thetextured surface 11 a of the rear glass substrate 11 so as to besubstantially continuous across substantially the entire surface 11 a ofsubstrate; any rear electrode 10 discussed herein may be used (S2).Optionally, after the rear electrode 10 has been formed on the rearglass substrate 11, the major surface 10 a of the rear electrode 10 tobe closest to the light absorption semiconductor film 5 may be texturedvia one or more of ion beam treatment, plasma exposure, and/or a wetchemical etching such as HCl, nitric acid, and/or acetic acid or acombination thereof (S3). Again, if ion beam treatment is used in stepS3, example ion sources and ion beam treating techniques are describedin U.S. Pat. Nos. 7,049,003 and 6,878,403, which are incorporated hereinby reference. After step S3, the rear electrode 10 may still besubstantially continuous across substantially the entire surface 11 a ofthe substrate. After the formation of the rear electrode 10 has beencompleted, then in certain example instances the semiconductor film 5and front electrode 3 may then be deposited on the glass substrate 11over the roughened rear electrode 10. Then, the front substrate 1 may belaminated to the rear glass substrate 11 via adhesive 2 or the like. Therear electrode structure is thus used in a photovoltaic device (S4).

FIG. 7 is a flowchart illustrating steps taken in making a photovoltaicdevice according to another example embodiment of this inventionrelating to textured rear substrates as shown in FIGS. 1( b), 2, 3(a), 4and 5. This embodiment is the same as the FIG. 6 embodiment, except thatthe surface 11 a of the rear glass substrate 11 need not be textured. Inthe FIG. 7 embodiment, rear glass substrate 11 (e.g., soda-lime-silicabased glass) is provided (textured or not textured). Then, the rearelectrode 10 is deposited (e.g., via sputtering or the like) on a majorsurface of the rear glass substrate 11 so as to be substantiallycontinuous across substantially the entire surface of the substrate(SA); any rear electrode 10 discussed herein may be used. After the rearelectrode 10 has been formed on the rear glass substrate 11, its majorsurface to be closest to the semiconductor may or may not be textured.The major surface 10 a of the rear electrode 10 to be closest to thelight absorption semiconductor film 5 is then textured via one or moreof ion beam treatment, plasma exposure, and/or a wet chemical etchingsuch as HCl, nitric acid, and/or acetic acid or a combination thereof(SB). Again, if ion beam treatment is used in step SB, example ionsources and ion beam treating techniques are described in U.S. Pat. Nos.7,049,003 and 6,878,403, which are incorporated herein by reference.After SB, the electrode 10 may still be provided across substantiallythe entire surface of substrate 11. Then, in certain exampleembodiments, the semiconductor film 5 and front electrode 3 may then bedeposited on the glass substrate 11 over the roughened rear electrode10. Then, the front substrate 1 may be laminated to the rear glasssubstrate 11 via adhesive 2 or the like in forming a photovoltaicdevice. The rear electrode structure is thus used in a photovoltaicdevice (SC).

Referring to FIGS. 10-12, further example embodiments of this inventionwill now be described. The embodiments discussed below in connectionwith FIGS. 10-12 may (or may not) be used in connection with any of theother embodiments such as illustrated in FIGS. 1-9.

Cupper-indium-diselenide (CIS) and cupper-indium-gallium-diselenide(CIGS) based photovoltaic devices (e.g., solar cells) are good withrespect to efficiency among thin-film solar cells, in excess of 17%approaching that of crystalline silicon devices. A CIS and/or CIGSdevice (which may be referred to herein as a CIS device) is commerciallyfabricated to include a molybdenum (Mo) based rear electrode layer on aglass substrate which is then patterned (or scribed) by a laser to formindividual contacts or electrodes. The quality of the laser scribe isimportance to ensure high module efficiency. Thereafter, the rest of thestack is deposited, including for example the CIS layer, CdS layer, anda transparent conductive oxide such as tin oxide or zinc oxide as thefront electrode. An ordered defect chalcopyrite (ODC) can form at theCIS/CdS interface. Solar light enters the device through thewide-bandgap front transparent electrode (e.g., ZnO) and induces theelectron-hole pair generation in the CIS absorber film. The positive andnegative charges are divided by the electric field and exit the devicethrough the top and bottom electrodes.

It is desirable for the Mo based rear electrode 10 to possess certainqualities in order to serve as a good electrode. In certain exampleembodiments, rear Mo based electrodes 10: have a low sheet resistance(typically less than 1 Ω/sq) to effectively conduct the extracted holes(e.g., for this a certain grain size and grain structure should beachieved); have a low contact resistance to the CIS absorber; areeffective diffusers of sodium from soda-lime glass substrate 11 to theabsorber 5 during the device-making high-temperature processing (this isbeneficial for the efficient large-grain CIS growth); have asufficiently rough surface to promote a better adhesion of the absorber5 to the contact 10; have a proper work function (WF) to provide anohmic contact for the extracted holes; and ensure both good adhesion tosoda-lime glass 11 and high quality of the laser scribe.

Room-temperature magnetron sputtering (of Mo or MoOx based targets) inAr gas has proved successful as an effective way of Mo deposition forsuch electrodes 10. However, a disadvantage of a single-layerconfiguration of Mo for the rear electrode is its low sodiumpermeability. Furthermore, large-area Mo deposition is often desired todrive the cost of the module down.

In order to address the above issues, in certain example embodiments ofthis invention there is provided a method of making the rear electrode10 for CIS and/or CIGS photovoltaic (e.g., solar cell) devices usingmagnetron sputter-deposition of molybdenum (Mo) in a multi-layerconfiguration. In certain example embodiments, nitrogen and/or hydrogengases are used as additives to the sputtering gas (e.g., argon) toreduce stress of the rear electrode coating, enhance its resistance tothe selenization during the downstream device-making processing, andpromote beneficial sodium migration from the soda-lime rear glasssubstrate 11 to the semiconductor film 5 of the device.

Referring to FIGS. 10-12 for example, certain example embodiments ofthis invention use the following steps in the fabrication of a Mo basedrear electrode 10: (i) addition of nitrogen and/or hydrogen to thesputtering gas during the Mo deposition; (ii) use of multi-cathodesputter deposition; (iii) use of a buffer layer 10 b (e.g., includingMoO_(x), so as to have more oxygen than other areas of the Mo based rearelectrode) between the rear glass substrate 11 and more conductiveportions of the rear electrode 10; and (iv) use of an intermediatestress-relieving layer 10 c′ between the buffer layer 10 b and otherportion(s) 10 c of the Mo based rear electrode 10.

The use of hydrogen gas when sputtering Mo for the Mo-based rearelectrode 10 (e.g., in the context of layers 10 c and 10 c′, andpossibly 10 b), has multiple roles. For example, it has been found thathydrogen is advantageous for neutralization of oxygen from the growingMo surface. This increases the mobility of the arriving Mo atoms andenhances quality of the film and overall electrode 10. The use ofhydrogen gas during the Mo sputtering is also advantageous in that itincreases the Mo based electrode 10's resistance to selenization duringthe downstream CIS fabrication process (when the semiconductor film(s)is/are being formed), and reduces stress asymmetry between the substratetravel and cross-coater direction observed in large-area coatings.

The use of a MoOx based buffer layer 10 b between the rear glasssubstrate 11 and the layers 10 c and 10 c′ is advantageous for thereasons explained above in connection with layer 10 b. A significantpurpose of a buffer layer 10 b between the Mo (10 c, 10 c′) andsoda-lime-silica based glass 11 is to improve adhesion of the coating 10to the glass 11, and to facilitate the laser scribe. This is done by theaddition of oxygen and/or nitrogen during sputtering of the layer 10 b,and possibly by increasing the sputtering pressure. These two measurespromote the formation of a less crystalline film with better adhesionproperties. Grain size of the buffer 10 b is typically less than about20 nm in certain example embodiments of this invention. The optionalsecond stress-reducing layer 10 c′ is deposited under pressuresintermediate between the high pressure of the main more conductive Mofilm 10 c and the low pressure of the buffer film 10 b. Layer(s) 10 c(and possibly 10 c′) are more metallic than layer 10 b, because there isless oxygen in 10 c (and possibly 10 c′) than in 10 b in certain exampleembodiments of this invention.

Moreover, although oxygen is considered to have a negative effect onconducting properties of the film, slight oxidation between the cathodeswas found to be beneficial for the coating since small amounts of oxygencan promote desirable sodium (Na) diffusion from the glass 11 throughthe Mo and toward or into the semiconductor during downstream CISprocessing.

In certain example embodiments of this invention, referring to FIG. 10for example, one or two sputtering targets may be used tosputter-deposit MoOx or Mo:NOx buffer layer 10 b on (directly orindirectly) glass substrate 11. Metallic Mo or ceramic MoOx sputteringtarget(s) may be used in depositing buffer layer 10 b. Likewise, one ortwo sputtering targets may be used to sputter-deposit optional Mo:N(H)intermediate layer 10 c′. More hydrogen gas (and less oxygen gas) isused when sputter-depositing layer 10 c′ than in depositing layer 10 b.Little or no hydrogen is used when sputter-depositing layer 10 b incertain example embodiments. Moreover, one, two, three, four or moresputtering targets may be used to sputter-deposit Mo:N(H) substantiallymetallic Mo-based layer(s) 10 c. Thus, layer 10 c may be made up of oneor more layers in example embodiments of this invention. Less (or no)oxygen gas is used when sputter-depositing layer 10 c than in depositinglayer 10 b and/or 10 c′. Metallic or substantially metallic Mo targetsare used in sputter-depositing layer 10 c and 10 c′ in certain exampleembodiments. Residual nitrogen, oxygen and hydrogen from these gasesused during sputtering of electrode 10 may end up in the actual layers10 b, 10 c′ and 10 c in the final electrode.

It has been found that further promotion of desirable sodium migrationis achieved by using nitrogen during the Mo sputtering of rear electrode10, in order to slightly amorphize the Mo-based electrode 10 withoutsignificantly affecting its electrical performance. In this respect,nitrogen gas may be used during the sputtering of layers 10 c′ and/or 10c, and possibly even layer 10 b (this applies to any embodiment herein).As shown in FIG. 11, resistivity of nitrogen-doped Mo-based films wasfound to have a minimum at around about 1 wt. % of nitrogen in Ar gasused in the Mo sputtering. As also shown in FIG. 11, at about 0-2 wt. %of nitrogen (e.g., N₂) gas in the overall Ar gas during sputtering, theMo film is essentially purely metallic and demonstrates a predominant<110> orientation. At >2 wt. % of nitrogen gas in the Ar gas, thepresence of Mo2N with an <112> orientation can also be detected.

Besides promoting the sodium migration from soda-lime glass 11 to theCIS absorber 5, nitrogen used in sputtering and in film 10 is alsoadvantageous in that it stabilizes the Fermi Level (FL) disturbed by aslight oxidation between the sputtering regions (see FIG. 12). The banddiagram of the device with the energy position of corresponding bands inrespect with the vacuum level is shown in FIG. 12. Work function of Mois known to vary between 4.2 and 5.0 eV depending on the depositionconditions. For instance, Na implantation is used to lower WF of Mo inn-FETs for CMOS devices. On the other hand, oxidation of Mo at theinterface with monoclinic zirconia is known to increase or decrease WFdepending on the level of oxidation due to the rearrangement of theinterface dipoles. The positioning of the FL is usually a trade-offbetween having a good ohmic contact properties and high electricalconductivity. Depending on the specifics of the CIS device fabrication,particularly, the composition of the absorber 5, adjustment of the FL ofthe bottom electrode 10 may be utilized. The use of nitrogen, therefore,is a convenient tool in tuning the Mo properties of the rear electrode10 to a particular photovoltaic device.

In certain example embodiments of this invention, argon (Ar) or someother inert gas is the primary gas used in sputter-depositing theMo-based layers of rear electrode 10. However, for the reasons explainedabove, in certain example embodiments, buffer layer 10 b issputter-deposited using a Mo or MoOx target(s) in an atmosphereincluding from about 0.001-10% (by weight) nitrogen gas, more preferablyfrom about 0.1 to 3% nitrogen gas, even more preferably from about 0.5to 1.5% nitrogen gas, and most preferably about 1% nitrogen gas, withthe remainder of the gas in the sputtering atmosphere for this layer 10b being made up mostly or entirely of argon (or some other inert gas)and possibly a small amount of oxygen gas as discussed herein. In thisrespect, buffer layer 10 b is based on Mo but includes from about0.001-10% nitrogen, more preferably from about 0.1 to 3% nitrogen, evenmore preferably from about 0.5 to 1.5% nitrogen, and most preferablyabout 1% nitrogen, in certain example embodiments of this invention.Moreover, in certain example embodiments of this invention, buffer layer10 b is sputter-deposited using the Mo or MoOx target(s) in anatmosphere including (in addition to the argon and possibly the nitrogengas discussed above) from about 0.001-10% (by weight) oxygen gas, morepreferably from about 0.1 to 3% oxygen gas, even more preferably fromabout 0.5 to 1.5% oxygen gas, and most preferably about 1% oxygen gas,with the remainder of the gas in the sputtering atmosphere for thislayer 10 b being made up mostly or entirely of argon (or some otherinert gas) and possibly a small amount of nitrogen gas as discussedabove. In this respect, buffer layer 10 b is based on Mo but includesfrom about 0.001-10% oxygen, more preferably from about 0.1 to 3%oxygen, even more preferably from about 0.5 to 1.5% oxygen, and mostpreferably about 1% oxygen, in certain example embodiments of thisinvention. The oxygen content of layer 10 b may or may not be graded asdiscussed herein. Moreover, the Mo-based buffer layer 10 b has anaverage grain size in diameter of from about 0.1 to 30 nm in certainexample embodiments, more preferably from about 1-20 nm, and mostpreferably from about 5-20 nm.

In contrast, the Mo-based intermediate layer 10 c′ has a larger averagegrain size than does layer 10 b, and in certain example embodiments theintermediate layer 10 c′ has an average grain size in diameter of fromabout 5 to 50 nm in certain example embodiments, more preferably fromabout 20-50 nm, and most preferably from about 25-45 nm.

For the reasons explained above, in certain example embodiments,Mo-based layer 10 c′ and Mo-based layer 10 c (in any embodiment herein)may each be sputter-deposited using a Mo based target(s) (e.g., metallicMo) in an atmosphere including from about 0.001-10% (by weight) nitrogengas, more preferably from about 0.1 to 3% nitrogen gas, even morepreferably from about 0.5 to 1.5% nitrogen gas, and most preferablyabout 1% nitrogen gas, with the remainder of the gas in the sputteringatmosphere(s) for these layer(s) 10 c and 10 c′ being made up mostly orentirely of argon (or some other inert gas) and possibly some hydrogengas as discussed herein. Moreover, Mo-based layer 10 c′ and Mo-basedlayer 10 c (in any embodiment herein) may each be sputter-depositedusing a Mo based target(s) (e.g., metallic Mo) in an atmosphereincluding from about 0.001-10% (by weight) hydrogen gas, more preferablyfrom about 0.1 to 3% hydrogen gas, even more preferably from about 0.5to 1.5% hydrogen gas, and most preferably about 1% hydrogen gas, withthe remainder of the gas in the sputtering atmosphere(s) for theselayer(s) 10 c and 10 c′ being made up mostly or entirely of argon (orsome other inert gas) and possibly some nitrogen gas as discussedherein. Small amounts of oxygen may also be present, as there can beslight intentional or unintentional oxidation between adjacentsputtering regions due to multi-cathode deposition. In certain exampleembodiments, Mo-based layer 10 c′ and Mo-based layer 10 c (in anyembodiment herein) may each be sputter-deposited in an atmosphereincluding from about 0.001-10% (by weight) hydrogen and nitrogen gascombined, more preferably from about 0.1 to 3% hydrogen and nitrogen gascombined, even more preferably from about 0.5 to 1.5% hydrogen andnitrogen gas combined, and most preferably about 1% hydrogen andnitrogen gas combined, with the remainder of the gas in the sputteringatmosphere(s) for these layer(s) 10 c and 10 c′ being made up mostly orentirely of argon (or some other inert gas). These gas percentages areweight percentages. The technique of tuning the Work Function of themulti-layer Mo based electrode 10 to provided a good Fermi level mayalso be used to match or substantially match the CIS absorber.

While the Mo-based electrodes discussed herein (which may be of othermetal in certain alternative embodiments) are used as rear electrodes ofphotovoltaic devices, this invention is not so limited. In particular,this invention may be used to form electrodes in such a manner for otherapplications as well.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1-33. (canceled)
 34. A photovoltaic device comprising: a frontsubstrate; a front substantially transparent conductive electrode; anabsorber semiconductor film; a conductive and reflective rear electrode;a rear glass substrate that supports at least the rear electrode; andwherein the rear electrode comprises a first layer or layer portioncomprising an oxide of Mo and a second conductive layer or layer portioncomprising substantially metallic Mo provided on the rear glasssubstrate over at least the first layer, so that the first layer orlayer portion comprising the oxide of Mo is located between the rearglass substrate and the second layer or layer portion comprisingsubstantially metallic Mo.
 35. The photovoltaic device of claim 34,wherein the semiconductor film comprises CIS and/or CIGS.
 36. Thephotovoltaic device of claim 34, wherein a surface of the rear electrodeclosest to the semiconductor film is textured so as to increasereflective scattering by the rear electrode.
 37. The photovoltaic deviceof claim 34, further comprising a layer comprising Ti and/or Cr locatedbetween at least the rear glass substrate and the first layer or layerportion comprising an oxide of Mo.
 38. The photovoltaic device of claim34, wherein one or both of (i) the first layer or layer portioncomprising an oxide of Mo and (ii) the second conductive layer or layerportion comprising substantially metallic Mo, comprises from about 0.1to 3% hydrogen and/or nitrogen.
 39. The photovoltaic device of claim 34,wherein each of (i) the first layer or layer portion comprising an oxideof Mo and (ii) the second conductive layer or layer portion comprisingsubstantially metallic Mo, comprise from about 0.1 to 3% hydrogen and/ornitrogen.
 40. A method of making a rear electrode structure for aphotovoltaic device, the method comprising: providing a glass substrate;depositing a conductive electrode comprising Mo (molybdenum) on theglass substrate; and wherein said depositing the conductive electrodecomprising Mo (molybdenum) comprises sputtering at least one targetcomprising Mo in an atmosphere including (i) an inert gas, and (ii) fromabout 0.1 to 10% nitrogen and/or hydrogen gas.
 41. The method of claim40, wherein said depositing of the conductive electrode comprising Mocomprises sputtering at least one target comprising Mo in an atmosphereincluding from about 0.1 to 10% nitrogen gas.
 42. The method of claim40, wherein said depositing of the conductive electrode comprising Mocomprises sputtering at least one target comprising Mo in an atmosphereincluding argon gas and from about 0.1 to 3% nitrogen gas.
 43. Themethod of claim 40, wherein said depositing of the conductive electrodecomprising Mo comprises sputtering at least one target comprising Mo inan atmosphere including from about 0.1 to 10% hydrogen gas.
 44. Themethod of claim 40, wherein said depositing of the conductive electrodecomprising Mo comprises sputtering at least one target comprising Mo inan atmosphere including argon gas and from about 0.1 to 3% hydrogen gas.45. The method of claim 40, further comprising sputter-depositing abuffer layer comprising an oxide of Mo on the glass substrate, whereinthe buffer layer is located directly on and contacting the glasssubstrate.